1. Field of the Invention
The present invention relates to an oversampling FIR filter for use in a portable terminal, etc. of a mobile communication system.
2. Description of the Related Art
FIR (Finite Impulse Response) filters have such linear phase characteristics, transfer functions, and stability that analog filters cannot achieve, and thus are put into a variety of uses including communication apparatuses and audio apparatuses. In particular, FIR filters are effectively applied to the filters on CDMA (Code Division Multiple Access) and other digital mobile communication methods. For stabilization of filter rejection regions, FIR filters of oversampling type have become dominant.
Among the oversampling FIR filters of this type is, for example, the one disclosed in Japanese Unexamined Patent Application Publication No. Hei8-37444.
The oversampling FIR filter disclosed in the publication includes: a shift register having a plurality of delay elements for holding input data; a factor selector; and an adder. The factor selector generates predetermined tap factors in accordance with the output values from the delay elements of the shift register and a timing signal. The adder obtains the sum of the plurality of tap factors output from the factor selector.
In an oversampling FIR filter configured as described above, the shift register need not be provided with as many delay elements as the number corresponding to the number of oversamples of the input data. This reduces the adder in scale, allowing a simpler hardware configuration as compared with ordinary oversampling FIR filters. Incidentally, in the following description, an oversampling FIR filter of this type will be also referred to as an oversampling FIR filter of tap switching type.
In the CDMA method, a portable terminate diffuses its to-transmit signals with diffusion codes and output the diffused data toward a base station. Here, for the sake of phase adjustment of data received at the base station, the portable terminal needs to change the number of oversamples in its FIR filter depending on the distances between the portable terminal and the base station. The above-mentioned FIR filter of tap switching type does not have as many delay elements as the number corresponding to the number of oversamples of input data. The number of oversamples signifies the number of times oversampling is performed. Accordingly, a change in the number of oversamples has caused a problem of discontinuity in output response. Thus, it has been impossible for FIR filters of tap switching type to be used for portable terminals in the CDMA method with varying the number of oversamples. Meanwhile, oversampling FIR filters having as many delay elements as the number of oversamples are greater in circuit scale as compared with the oversampling FIR filters of tap switching type. However, the number of zero data to be added to input data can be changed in accordance with the number of oversamples, thereby achieving the continuity in output response.